I tried fixing other tags: if any tags are different, iTunes may list the albums as different, in spite of their having the same name. What finally resolved the problem was deleting the Sort Artist and Sort Album Artist tags. (Select all the tracks, press Command-I, then click the Sorting tab; remove the check mark in the boxes next to Sort Artist and Sort Album Artist.) This fixed the problem. The cause was most likely a different name in one of those boxes (since I had selected multiple items, it didn't show if there was a difference; I could have checked first, but didn't.)
In any case, if you've normalized all your visible tags and albums still don't stay together, try checking the Sorting tags. You won't see them in iTunes, but iTunes uses them to sort and organize albums.

